1. Field of the Invention
The present invention relates to object location monitoring within or around buildings. More particularly, the present invention relates to systems and methods for monitoring the locations of transceivers or transponders that may be attached to a patient or other movable object.
2. Description of the Related Art
Various approaches have been proposed for monitoring the locations of objects within an area. One such approach uses radio frequency (RF) transmitters or tags, attached to the objects to be tracked, and an array of receivers for receiving tag transmissions throughout a tracking area. The signal strengths of the received tag transmissions may then be used to determine a tag""s distance from the receivers. One problem with this approach is that it is not well suited for monitoring the locations of objects within a building having multiple rooms. In such an environment, the unpredictable attenuation of transmissions passing through walls and other structures, and the effects of multi-path distortion, make signal strength only marginally useful as a parameter for determining distance.
One solution to this problem, proposed in U.S. Pat. No. 5,119,104 to Heller, involves accurately measuring the time of arrival of the tag transmission at each of multiple receivers. This time of arrival data is then used in combination to determine the location of the tag. A significant problem with this approach is that a high degree of synchronization is required between the receivers to produce accurate results. As a result, such a system tends to be difficult and expensive to implement.
In accordance with the invention, an object location monitoring system tracks the locations of portable, wireless transceivers that attach to movable objects, such as patients within a hospital. The system preferably tracks the locations of the transceivers by combining a signal strength based locating process with a topological tracking process. The transceivers can be any type of data processing device capable of sending and receiving data by RF, including transponders.
In a preferred embodiment, chirpers or xe2x80x9cbeaconsxe2x80x9d that intermittently transmit unique ID codes are mounted to walls or other structures throughout areas of a building. The beacons are preferably maintained in coarse synchronization such that nearby beacons do not transmit simultaneously. In accordance with one aspect of the invention, the beacons are synchronized using the 60 Hz cycle of a standard 120V AC (alternating current) power signal within the buildingxe2x80x94either by monitoring the power signal directly or by monitoring the flicker of the lighting within the building. Two or more beacons are preferably provided in different locations within each room to reduce ambiguities and the effects of signal blockage.
Each transceiver records the beacon IDs, and determines the received signal strengths, of the beacon transmissions it detects. The received signal strength establishes a maximum plausible distance between the beacon and the transceiver, based on the assumption that the received signal strength decreases with at least the second power of the distance as in free space propagation. The transceivers preferably forward some or all of this information to a server or other processing node. The processing node uses this information, together with information about expected received signal strengths in specific areas, to predict the current location of each transceiver. Any ambiguities in the location data are preferably resolved using the topological tracking process. The topological tracking process preferably uses predetermined information about the minimum times an object may take to move from one location to another. The topologic tracking process may also use rules regarding plausible state transitions; for example, if a transceiver was very recently determined to be in room 1, and there is object locator coverage in the hallway that provides the only path from room 1 to room 2, the system may treat a direct transition from room 1 to room 2 (without intermediate detection in the hallway) as suspect or invalid.
In one particular embodiment, the transceivers are remote patient transceivers or telemeters used to monitor and transmit patient vital signs data within a medical facility. These devices are typically portable, battery-powered devices configured to be worn by respective patients. In a TDMA (time division multiple access) implementation, each patient transceiver transmits the patient""s physiologic data to a receiving station according to a TDMA protocol. Each patient transceiver additionally listens for beacon transmissions during TDMA time slots that are not otherwise used by that patient transceiver, and records the beacon IDs and received signal strengths of the detected beacon transmissions. The patient transceiver forwards some or all of this information about the received beacon transmissions to a server or other processing node preferably using a link established with an access point for telemetry purposes. The processing node uses this information together with a set of tables to determine the transceiver""s location.
Although the process used to determine the locations of the transceivers is preferably executed by a central server or processing node, the process could alternatively be implemented within the transceivers themselves (i.e., each transceiver determines its own respective location), or within network access points with which the transceivers communicate.